1. Field of the Invention
The present invention relates to an optical coupling device used for an optical wavelength converter comprising a waveguide of core and a clad surrounding the core for converting a wavelength of a fundamental wave entering the core to a 1/2 wavelength of a secondary harmonic, and more particularly to an optical coupling device for coupling optically a semiconductor laser to an end surface of the functional three-dimensional waveguide of the wavelength converter.
2. Description of the Related Art
There is an optical wavelength converter system of waveguide-type as shown in FIG. 1. This three-dimensional waveguide type wavelength converter 5 utilizes a semiconductor laser 1 as a light source generating a fundamental wave of a light beam. The optical wavelength converter 5 comprises a core made of a nonlinear optical material and a clad substrate 3 surrounding the core. A converging lens 2 placed between the semiconductor laser and the converter converges the fundamental wave into the rectangle end surface of the core which is an optical coupling portion. The converter 5 converts the wavelength of the fundamental wave to a 1/2 wavelength of secondary harmonic.
Generally, the thickness and width of the three-dimensional waveguide are different from each other in an active region of the semiconductor laser. The laser light emitted from the end surface of the active region is diverged by the effect of diffraction in which the divergence angle in the wide direction is smaller than that of the thickness direction. Therefore, the emitted laser light becomes a diverging light having divergence angles different each other in the magnetic field direction and the electric field direction which has a transverse section of an elliptic shape and a different power distribution in the longitudinal section. The emitted laser light is linearly polarized as a TE mode which is parallel to the junction plane in the active region of the semiconductor laser.
It is necessary for a high efficiency wavelength conversion in the three-dimensional waveguide to couple a diverging laser light of fundamental wave emitted from the semiconductor laser into the three-dimensional end surface of the optical wavelength converter at a high coupling efficiency. In order to achieve such an efficient optical coupling, the position of a converged light spot of a fundamental wave emitted from the semiconductor laser 1 is accurately coincided to a rectangle end surface of the three-dimensional waveguide in a reference optical axis Z direction by means of a mechanism of three-dimensional relative position alignment in both the XY directions on the rectangle end surface and the reference optical axis Z direction, as shown in FIG. 1.
By the way, there are methods for entering a laser light into the waveguide of the waveguide type wavelength converter which employ a single converging lens above, a collimator lens making a parallel laser light, a pigtail fiber, a Selfoc (registered trade mark) lens, a rod lens and a combination of a halfwave plate placed between two convex lens.
There is a problem in the coupling method using a simple convex lens converging a laser light emitted from the semiconductor laser such that the rectangle shape of the input end surface of the three-dimensional waveguide does not meet with an elliptic shape of the converged laser light spot, resulting in the low coupling efficiency.
For example, even when a diverging laser light having divergent angles of 10.degree. in the electric field direction and 30.degree. in the magnetic field direction emitted from a semiconductor laser is coupled to a rectangle end surface of a symmetrical three-dimensional waveguide with 7 micrometer square section under a pertinent condition to a high coupling efficiency, as shown in a solid line in FIG. 2, the rectangle end surface 3a of the three-dimensional waveguide does not meet with the elliptically converged laser light spot S, resulting in a low coupling efficiency of about 55%. If the elliptic light spot S is expanded as shown in a dotted line in FIG. 2, both the ends of the elliptic light spot is out from the rectangle end surface to waste light energy.
In this case, the far field pattern of the diverging laser light should be deformed to a circle in order to improve the coupling efficiency between the diverged laser light with an ellipse section and the a rectangle end surface of the three-dimensional waveguide. Such a beam shaping method utilizes a collimator lens for making a parallel beam, and a beam shaping prism for increasing or decreasing a short or long axis of an ellipse transverse section of the beam. However, the increase of the prism member and the difficulty of the position alignment for these optical members occur as a problem which is an obstacle to minimize an optical wavelength converter.
On the other hand, when the optical coupling is performed by a coupling method using a pigtail fiber which is not easily manufactured, a plane of polarization of the fundamental wave passing through the pigtail fiber is irregular. This condition is insufficient to improve the performance the functional three-dimensional waveguide of the wavelength converter.
In addition, these conventional coupling methods bring a high cost for production of the optical wavelength converter.